1. Field of the Invention
The present invention relates to electromagnetic flow meters.
2. Description of Related Art
Electromagnetic flow meters have been used for many years for measuring the flow of a conductive fluid. Electromagnetic flow meters have a number of advantages over mechanical meters as they can be made relatively compact and without moving parts to wear or become damaged by inclusions within the fluid. Electromagnetic flow meters work on the principle that a magnetic field (which may be static or time-varying) induces an EMF across a moving fluid. As a practical matter however, the EMF is typically small and measuring it with practical instruments without undue sensitivity to noise is a concern.
It conveniently so happens that water-based fluids, which constitute a usefully large proportion of process fluids to be monitored, tend to have a conductivity that can be measured using an electromagnetic flowmeter of convenient dimensions. In fact, pure (demineralised) water has a relatively low conductivity and this may be problematic to measure as the impedance of a practically sized meter would be very high. However, measurement of pure water is rarely called for as practical fluids have much higher conductivities, tap water having a conductivity approximately 100 times greater than pure water and seawater having a conductivity approximately 10,000 times greater. Typical water-based process fluids have conductivities falling between the latter two. Thus, for (impure) aqueous fluids, electromagnetic flow meters can conveniently be manufactured with practical dimensions.
Non-polar, “insulating”, fluids such as oil-based fluids are not conventionally considered suitable for use with electromagnetic flow meters and there are a wide variety of other metering techniques adapted for such fluids.
The reason for this can best be understood by considering typical conductivity values. Fluids such as turpentine have conductivities of the order of 2,500 times lower than that of tap water and 200,000 times lower than seawater. Alcohols typically have similar conductivities (or slightly higher) and fluids such as kerosene and oils have still lower conductivities.
Accordingly, whilst electromagnetic flowmeters have become widely used for measuring the flow of impure aqueous fluids, other techniques are indicated for “non-conductive” fluids, e.g. organic solvents, turpentine, alcohol or oil-based fluids.